The 1e Monster Manual displays a remarkable focus on mundane animal ecology. Almost a third of "monsters" in the MM are just plain old animals, some in giant form, a small handful with some add-on power, like killer frogs and fire toads.
Has anyone ever used a giant weasel in their campaign? I think, probably, no, no one has EVER included a giant weasel in a D&D game... But that is not the point!
The point is that the MM was designed to enable you to stock a holistic campaign, i.e., include life outside the dungeon, or even the town. Alright, fine, but outside of stocking random wilderness encounter tables, what else are they good for?
Adventure Hooks involving animals:
--Bad guy henchmen: evil wizard/warlord living in that dark forest/mountain lair, seems to somehow control an army of vicious animals, to act as his defense and spies on the approach to his hidden fortress.
--Gladitorial combat: a) Ending up on the wrong side of the law after a night of post-dungeon partying, PCs are sentenced for a night of Idiocracy-style "rehabilitation", on the stadium floor with a troop of hungry wolverines and cave bears....
--Gladitorial combat: b) down-on-their luck PCs turn to a local help wanted ad, for a high-wage position. The ad only mentions one caviat, that "applicant must feel comfortable working with animals"...
--Resource acquisition: animals often have products that are highly valuable, which the perpetually poverty-stricken PCs might get involved with, such as:
- the royal jelly in giant's bees nests, that the royal princess demands and daddy/king pays a pretty penny for...
- the poison in giant scorpion stingers, as one of the street kids has a friend, who has a friend, who is willing to pay top dollar for the pure stuff, don't mention it to the town guards on the way in, though... no, nothing at all to do with the assassin's guild, of course not, don't be silly....
- eggs of various creatures, especially flying creatures, that the nobles are always trying to train as steeds or as aerial hunters or messengers.
- babies of various species, to be captured and raised as pets and/or zoo specimens by eccentric nobles, or guard animals by merchants, such as young bears, tigers, hyenas, wolves, crocodiles, etc.
--Bounty rewards: local villagers/farmers/ranchers post a bill offering rewards for the elimination of animal threats. "Looks like a hive of gaint wasps just popped up on the other side of that hill, in that hole in the side of hill. You guys clear the nest for us, we'll pay you 1000 gold...." or "Stupid wolf pack has been getting real aggressive lately, attacking my sheep, 50 gold for each wolf hide you bring in..."
--Hunting fodder: PCs will often need to eat while traveling cross country, needing to hunt and battle/kill the animals for food. Keep in mind, this will always attract alpha predators, who hear the dying cries and/or smell the spilled blood, and decide to come get a share for themselves.